The spinal column is a flexible column formed from a linear series of vertebral bones separated by intervertebral discs. These discs reduce friction between adjacent vertebrae and absorb compression forces applied to the spinal column. A vertebra includes an anterior body and a posterior arch that surrounds the spinal cord. Spinal nerves extend from each side of the spinal cord and exit the column at the vertebral foramen, which is formed by the posterior arch. Articular processes, including the superior articular process and the inferior articular process, are small flat projections on the surfaces of the arches.
There are four facet joints associated with each vertebrae, and these joints interlock with adjacent vertebrae. In this manner, facets on the opposing processes determine the range and direction of movement between adjacent vertebrae, hence the flexibility of the spinal column. The facet joints maintain spinal stability, protect the disc from excessive stress, and assist the discs in allowing motion and controlling shear forces. These joints are vulnerable to degenerative spinal disorders.
Degenerative disc disease is typically caused by a loss of disc space height, leading to a narrowing of the neural foramen and subsequent neural compression, and causing back and radicular pain. Instability of the posterior elements can lead to a condition known as spondylolisthesis, in which a vertebral body slips forward in relation to an adjacent vertebrae. This movement of the vertebral body narrows the foramen and results in painful pressure on the nerve roots.
Degenerative disc disease may be resolved through a spinal fusion procedure using an interbody implant (which is implanted between the bodies of two adjacent vertebrae). Such interbody implants may be formed from titanium, carbon fiber, allograft, or other suitable material including, but not limited to, biocompatible materials such as PEEK (Polyetheretherketone)™. Implantation of a substitute graft is designed to reestablish normal disc height, provide immediate stability to the motion segment, and provide a matrix for fusion. When the implant grows into the existing bone, the fusion becomes solid and movement is eliminated at that level. A fusion procedure may also involve the surgical implantation of hardware, such as plates, screws or cages.
In order to fuse and thereby stabilize the motion segment, the disc space must be prepared prior to insertion of the interbody device. Soft tissue, such as disc material and cartilage, and other such tissue is cleaned off the vertebral endplates so that intimate bony contact is obtained between the graft, implant and host tissue. The preparation of the disc space can be achieved with scrapers, curettes, rongeurs, drills, rasps and/or chisels. In preparing the disc space, it is important not to remove too much of the endplate in order to maintain structural integrity so that the interbody implant does not telescope into the vertebral body when normal axial loads are applied.
Posterior Lumbar Interbody Fusion (PLIF) is one surgical fusion technique used to treat degenerative lumbar disc disease. Proper distraction during a PLIF procedure must be achieved in order to gain compression of the implant. Proper distraction allows natural compression across the disc space via the annulus and other posterior elements. This compression delivered to the implant helps stabilize the implant, which avoids expulsion, and keeps the grafting material under stress, thus promoting faster fusion and bone healing.
Transforaminal Lumbar Interbody Fusion (TLIF), also referred to as an extended PLIF, was developed in response to problems associated with PLIF procedures. In the TLIF approach, the disc space is expanded by removing one entire facet joint, while a PLIF is usually performed on both sides of the facet, removing a portion of each of the joints. Removal of the entire facet joint improves visualization into the disc space, allowing removal of more disc material and insertion of a larger implant. Other procedures have been developed to provide anterior column support as well, including the Anterior Lumbar Interbody Fusion (ALIF) and extreme lateral interbody fusion techniques that access the vertebrae through the psoas muscle.